In modern communication systems, coherent detection is usually employed, which requires a receiver to be very accurately synchronized with the transmitter in both time and frequency. In most systems, key elements of the synchronization scheme are designed with only additive white Gaussian noise (AWGN) in mind. However, many emerging devices operate in unlicensed spectrum, e.g., the 902-928 Industrial Scientific and Medical (ISM) band, where many unpredictable forms of interference are consistently present. In such devices, these commonly-used synchronization schemes, designed for best operation under AWGN conditions only, may vastly under-perform in strong interference environments. Furthermore, modern air interfaces, e.g., those employing Orthogonal Frequency Division Multiplexing (OFDM), often lend themselves to powerful detection techniques that potentially provide extreme resilience against the most powerful of interferers. This potential immunity to interference, however, will never be harnessed unless the receiver is able to acquire and maintain synchronization in the presence of such interference. The invention described herein pertains to receivers, in general, and is described within the context of, but not limited to, an OFDM system employing antenna diversity. Furthermore, the invention is primarily directed to partial-band interference, which is the most-common form of interference encountered when operating in unlicensed spectrum such as the 902-928 MHz ISM band.
In order to demonstrate the problem solved by this invention, along with the capability of the invention itself, we first provide a basic description of the system model and its key components, as illustrated in the block diagram of FIG. 1. Note that the ideas disclosed within this document are not strictly limited to the system under consideration as those skilled in the art can easily realize after review of this disclosure.
In FIG. 1 data packets are generated and encoded into channel bits. An OFDM symbol mapper converts the encoded channel bits into N streams of complex channel symbols, where N is the number of transmit antennas, and maps them onto the OFDM time-frequency grid, while merging them with known channel symbols used for synchronization and channel estimation/training. An OFDM modulator converts the N streams of complex time-frequency symbols into N complex time-domain waveforms, using an inverse fast Fourier transform (IFFT) with cyclic prefix insertion. A channel function generates time-dispersive channel fading, which exhibits a frequency-selective gain, for the OFDM and interfering signals, producing an output consisting of M waveforms, where M is the number of receive antennas. The interfering signal is modeled as either a continuous wave (CW), or a complex Gaussian signal of specified bandwidth, center frequency, starting point, and duration. Note that each of the bandwidth, center frequency, starting point, and duration may be specified as random. A receiver filter removes out-of-band noise from the noisy received signal, before splitting the filtered signal off into two branches. In the diagram, the upper branch is the synchronization block, which is the primary focus of this disclosure. The synchronizer generates time and frequency offset information, which is essential for proper operation of the OFDM demodulator and ensuing blocks. The OFDM demodulator performs a fast Fourier transform (FFT) on blocks of samples that have been time and frequency corrected by the synchronizer. The FFT output is fed to a channel estimation block, which estimates the channel gains for the desired signal, as well as any other important characteristics of the received signal, which are then fed, along with the FFT output, to the detector block. Note that the channel estimator may be particularly sensitive to synchronization error, depending on its design. The detector block uses the M branches of the FFT output, and channel estimation information, to form a best estimate of the complex transmitted symbols. These complex symbol estimates are sent to a de-mapping function, which may make further measurements pertaining to the received signal, along with its primary function of converting the complex symbol estimates to bit-level estimates for the purpose of channel decoding.
A traditional synchronizer correlates against a known sync waveform embedded within the waveform, and adjusts symbol timing and sometimes frequency offset based on the results of this correlation. Another very commonly used form of coarse sync acquisition for OFDM, e.g. that proposed by Schmidl, embeds a repeated pattern into the OFDM waveform at the transmitter, and in the receiver, correlates the received signal against a delayed version of itself, in an attempt to detect this repetition. This method is very effective under AWGN conditions, since it allows a very wide frequency offset capture range, offers good initial frequency offset estimation, and is computationally very simple. In such traditional schemes, the fine timing sync is then achieved by correlating the received signal against the known sync waveform, after initial acquisition and frequency offset correction. While this method is effective under AWGN conditions, these typical methods perform very poorly under interference conditions, and in this case, are, by far, the limiting factor in determining the receiver's ability to reject partial-band interference.
In the case of frequency offset estimation, these common methods, which correlate the received signal against a delayed version of itself when estimating offset, are very easily thrown off frequency by any noise or interference which are colored. In addition, this correlation method is easily fooled into false detection when in the presence of colored noise or interference, potentially leaving the receiver in a perpetual state of confusion if colored noise is continuously present. Furthermore, the fine sync correlation against the known sync waveform is rendered ineffective and therefore useless whenever strong partial-band interference is present, since the desired signal is very easily over-powered.